In a hydrogen fuel cell, hydrogen is delivered to the anode, and oxygen is delivered to the cathode. At the anode, the hydrogen is oxidized to H+ ions, which travel to the cathode. Electrons from the oxidation reaction travel through an external circuit to the cathode power to a device connected to the fuel cell. At the cathode, the electrons reduce the oxygen, which then reacts with the hydrogen ions to form water molecules.                At the anode:H2→2H++2e−        At the cathode:2H++2e−+½O2→H2O        
Because hydrogen fuel cells produce water as a waste product, the use of these fuel cells results in fewer environmental concerns than batteries, which typically contain heavy metals and acids or strong bases. In addition, fuel cells consume fuels that are provided to the fuel cell only as needed. Thus, the life of a fuel cell is, at least in theory, unlimited since the fuel cell only requires fuel from an external source that can be replenished periodically.
One type of fuel cell is a proton exchange membrane (PEM) fuel cell, which operates at lower temperature and pressure ranges than some other types of fuel cells. In a PEM fuel cell, the hydrogen is catalytically split into protons and electrons at the anode side of the membrane electrode assembly. The newly formed protons permeate through the membrane to the cathode side. The electrons travel along an external load circuit to the cathode side of the membrane electrode assembly to create the current output of the fuel cell. The oxygen delivered to the cathode side of the membrane electrode assembly reacts with the protons permeating through the polymer electrode membrane and the electrons arriving through the external circuit to form water. PEM fuel cells are useful for applications wherein cleanliness, quiet, and compactness are desirable, such as for portable electronic devices.